Austenitic steel alloy



Dec. 20, 1960 c. H. ARMITAGE 7 2,965,478

AUSTENITIC STEEL ALLOY Filed Oct. 10, 1958 2 Sheets-Sheet 1 anwm vliofi Me o 9 ma X41 QMQW .Dec. 20, 1960 c. H. ARMITAGE 2,965,478

AUSTENITIC STEEL- ALLOY Filed Oct. 10, 1958 2 Sheets-Sheet 2 RANGE OVER WHICH TRANSFORMATION BEGINS RANGE OVER WHICH TRANSFORMATION TO NON-AUSTENITIC PRODUCTS IS COMPLETED UPON COOLING TEMPERATURE F s 5 g s O O O I I I I l I .OI .IO LO IQO IOQO IOOOD TRANSFORMATION TIME HRS.

g/wvkwl/Iiori 450 Who 2H fliimikdga. .4 X mrvmag Unite States PatentifC) AUSTENITIC STEEL ALLOY Charles H. Armitage, Wauwatosa, Wis., assignor to Allis- Chalmers Manufacturing Company, Milwaukee, Wis. I

Filed Oct. 10, 1958, Ser. No. 766,590

4 Claims. (Cl. 75-124) The present invention relates generally to the manufacture of alloy steels and more particularly to methods for making and the compositions of austenitic low manganese steels characterized by high wear and impact resistance, good tensile ductility, and the ability to rapidly work harden when in service.

According to the prior practice, the so-called Hadfields or austenitic manganese steels are generally chosen for applications in which both Wear and impact resistance are required. The austenitic steels are normally chosen because of their ductility at high hardness levels whereas the martensitic steels are generally brittle at the high hardness levels necessary to resis-t Wear.

Generally speaking, Hadfields manganese steels are ferrous alloys containing 1.0 to 1.4 percent carbon and 10.0 to 14.0 percent manganese as essential ingredients and are characterized by a stable austenitic structure at room temperature. In the middle of the above ranges, i.e., at about 1.2 percent carbon and 12-13 percent man ganese, Hadfields alloy steels exhibit their most desirable physical properties, including satisfactory tensile strength. When the manganese content is reduced, however, tensile properties rapidly decline to about one-half of their normal value at about 8 percent manganese. This and still further information regarding the known Hadfields or austenitic manganese steels is found at pages 526 et seq. in the Metals Handbook 1948 edition).

In actual use the austenitic manganese steels heretofore known have exhibited extremely poor wear resistance unless the surface is first work hardened by heavy impact blows.

One prior proposed improvement is austenitic manganese steels is described by Lorig in United States Patents Nos. 2,206,846 and 2,206,847. Lorig teaches an alloy which couples impact pounding with abrasion to transform an austenitic work surface into martensitic wear surfaces as rapidly as the earlier formed martensite surfaces are removed by abrasion.

More particularly, Lorig, as is described at page 533 of the Metals Handbook (1948 edition) in connection with US. 2,206,846 and 2,206,847, teaches a coppermolybdenum chromium bearing austenitic manganese steel having the following general composition: 1-5 per-' cent copper; up to 3 percent molybdenum; up to 3 percent chromium; 2.0-9.0 percent manganese; 0.501.7 percent carbon; up to 1.5 percent silicon; the balance substantially all iron. Experimental compositions near the mid-:

dle of Lorigs manganese range give tensile strengths of r 2,965,478 Patented Dec. 20,1960

ICC

68,000 to 88,000 p.s.i. with a 10-16% elongation. These values are similar to those of 9% manganese steel.

A number of disadvantages arise from the use of these materials. For example, in the use of the standard Hadfields steels, the heavy blOWs necessary to provide a proper work surface often cause extensive deformation of the soft austenite interior with the result that the parts so formed fall from loss of both shape and critical dimensions. And while Lorig somewhat reduces these functional disadvantages of the Hadfields'st'eel, Lorigs alloys possess further disadvantages of their own, including relatively inferior physical properties and the requirement that both copper and molybdenum be present to etfect the proper alloy.

Still a further disadvantage which characterizes all of the austenic alloy steels heretofore known is the inability to provide a machined austenitic steel end product b'ecause as soon as the material is worked by a tool, it hardens and prevents further working. p

The present invention proposes toovercome the disadvantages of the prior art austenitic manganese steels by introucing into the art new and improved methods for producing austenitic manganese steel alloys in a quick and eflicient manner and improved austenitic manganese alloy steels from which machined products can be formed. My improved alloys have a general chemical formulation of, in percent by weight: 0.8 to 1.5, carbon; 5.0 to 9.0, manganese; 2.0 to 5.0, chromium; 0.05 to 0.15, aluminum; with the remainder essentially iron; and are characterized by a matrix containing from 128 to about 1024 grains per square millimeter, a tensile strength of 100,000 to 140,000 p.s.i., an elongation of 16 to 44 percent, and a 20 to 46 percent reduction of area as produced and before being subjected to work hardening conditions.

Accordingly, one of the prime objects of the present invention is to provide an austenitic manganese steel alloy characterized by high wear resistance, ductility andhaving excellent work hardenability properties.

Another object of the present invention is to provide an improved austenitic manganese alloy steel which may be used for producing articles requiring a high sensitivity to work hardening and which need great strength and toughness.

Another object of the present invention is to provide'a work hardenable austenitic manganese alloy steel product having great resistance to both wear and impact and less deformation than the known prior art materials.

Still another object of the present invention is to provide an austenitic manganese steel having physical properties including a tensile strength of 100,000 to 140,000 p.s.i., an elongation of 16-44 percent (2 inch gauge), and a reduction of area of 20-46 percent, all of which are remarkably superior to known steels of this type; and having an empirical formulation totally devoid of copper, molybdenum, and special hardening agents such as vanadium, titanium, columbium, tungsten and the like.

An even further object of the present invention is to provide an improved method for preparing a leanly ala new leanly alloyed low manganese austenitic steel alloy characterized by physical properties superior to known austenitic manganese steel alloy containing 13-14 percent manganese and possessing an austenitic microstructure which at room temperature is thermally and mechanically unstable.

A still further object of the present invention is to provide an improved manganese austenitic steel alloy from which machined austenitic products can be readily formed.

These and still further objects as shall appear are fulfilled by the present invention in a manner easily discerned from, the following detailed description of articles and procedures exemplifying. the concept of the present invention, particularly when read in conjunction. with the accompanying drawing in which:

Fig. 1 isv a photomicrograph, enlarged one hundred times, of the microstructure of a cross section of an article formed from standard Hadfield7s austenitic manganese steel, and has a No. 1 A.S.T.M. grain size (0.19 to 0.37 grains per square inch of image);

Fig, 2 is a photomicrograph, enlarged one hundred times, of the microstructure of a cross section of an article formed in accordance with the present invention, and has a No. 5 A.S.T.M. grain size (12 to 24 grains per square inch of image);

Fig. 3. is an isothermal transformation diagram for the alloy steel of the present invention showing transformatlontime (hours) as ordinate and temperature F.) as abscissa; and

Fig. 4 is an isometric view of a suitable test bar for use in connection With the measurement of equivalent deformation herein described.

One important aspect of the present invention is predicated upon my discovery that an austenitic manganese steel of the Hadfields type can be prepared having superior properties (including tensile strength in excess of 100,000 p.s.i.) to those heretofore generally accepted as the maximum obtainable and a greatly refined grain structure While having a manganese content substantially lower than the percent heretofore specified s an absolute minimum etfective amount for the Hadfields type steel alloys. I have further found that by using particular quantities of chromium and aluminum as alloying agents that an austenitic mmganese steel. can be obtained in which the austenite structure is satisfactorily stabilized for the desired characteristics without requiring the formation of a completely stable austen-itic structure. Thus, contrary to the prior thinking on the subject, I obtain an excellent work hardenable. wear and impact resistant alloy steel having excellent tensile properties and an extremely fine grain structure with an austenitic manganese steel which contains less than 10 percent manganese, and more particularly from about 5' to 9.0 percent manganese, and which does not possess a completely stableaustenitic structure.

The general. composition range in. which the. specified advantages of my new alloy steel are readily attained comprises, in percentages by Weight:

Carbon 0.8 -l.5 Manganese 5.0 9.0 Chromium 2.0 -5.0 Aluminum 0.05-0.15 Iron Remainder The excellent mechanical properties of my alloy are achieved by the special balance of the aforesaid chemical composition and by my special heat treatment which I shall explain more fully. Though I do not desire to be held to a specific theory, I believe that the deliberate reduction of the stability of the austenitic structure and the remarkably refined grain structure in my alloy is responsible for the improved workhardening character-- istics of the alloy..

In, a ,more. preferred form, my -improved alloy, contains Y 18.0 to 44.0 percent (2 inch gauge) and have a reduction of area of 20.0 to 46.0 percent.

In the practice of one embodiment of my present invention, I use an electric furnace, although an open hearth or any other suitable steel furnace will serve equally well, heated to a temperature of from about 2900 to 3000 F. while introducing my iron charge. It has been my experience that almost any low carbon steel scrap containing 1 percent of carbon or less provides a suitable charge in the practice of this invention. Further, any combination of ferrous materials can be used which will not violate my final composition. I have also found that certain low silicon pig irons may be used provided they do not extend my final composition above the. normal maximums for silicon, sulfur and phosphorus prescribed by steel foundry practice for manganese steels, viz., 1.0 percent silicon and 0.04 percent sulfur and phosphorus. When the iron is completely molten, I then add my manganese and chromium in a suitable form such, for example, as granular ferromanganese and granular ferrochromium (which are the standard commercial forms of these materials). If the cross section of the pieces which I shall pour with my molten alloy is relatively large, i.e., greater than 5 to 7 inches, I may precede the addition of manganese and chromium to my molten iron with an addition of not more than 4 percent of nickel with satisfactory results. When the cross sections I desire to pour are of average size, i.e., less than about 7 inches thick, there is no appreciable advantage to be realized from adding nickel to the melt. I find that if my steel is to be used for nonabrasive applications, its stability may be increased by adding nickel in amounts up to about 4 percent. However, since nickel has a decidedly adverse effect upon the work hardenability of my alloy, its presence is tolerable in only small amounts, i.e., less than 4 percent even in the largest castings.

When the addition agents have been melted into the molten iron, I am ready to tp my furnace. I have found that the addition of aluminum such as aluminum shavings into the ladle either before or during the introduction of the molten. bath into t e ladle greatly enhances the physical properties of my alloy While at the same time serving to deoxidize my melt and provides a remarkable as-cast, grain refinement which persists throughout the subsequent heat treatment which I shall describe.

The molten alloy is next poured from the ladle into suitable molds made from patterns of thearticle which I desire to form and permitted to cool to about room temperature, that is, to less than 200 F., in accordance with normal foundry practice.

In the formation of my alloy steels, I have found that I can achieve equally satisfactory results when I introduce manganese as electrolytic m nganese and I can easily raise my carbon content by adding carbon either as stick graphite or in controlled amounts of high carbon pig iron.

After my molds have cooled, I introduce the castings so formed of my alloyed steel into an annealing furnace where I heat them to a temperature of about 1100 F; When my castings attain a temperature of about 1100 F., I hold them at this temperature for a period from 2 to 16 hours. After I heat my castings for the desired period of time, I permit them to cool very slowly usually, but not necessarily, in the furnace itself maintaining a gradual cooling rate, e.g., at about 50-150 F. per hour but not over 200 F. per hour.

When my steel alloy has reached a cool state, i.e., less than. 200 F. and preferably less than F. such for example. normal room temperature, itishard, brittle and very magnetic. Under the microscope, the microstructure is found to consist substantially of pearlite and bainite with some martensite interspersed therewith.

The isothermal transformation diagram which I have prepared for my alloy, and reproduced as Fig. 3, shows a maximum reaction rate at about 1100 F. although it is quite satisfactorily operable between 1050 to 1150 F. Temperatures on the other side of this range, for example 1000 or 1200 F., provide a much slower reaction and greatly increase the overall time and expense involved in the production of my improved alloy. During the course of the initial heating, the austenjte, through isothermal transformation, converts primarily to pearlite and becomes depleted in carbon, a primary austenite stabilizer.

During the transformation which occurs while holding the steel at 1050-1150 F., the carbon comes out of solution as carbide platelets. (It is believed that these carbide platelets are iron manganese carbide platelets.) After the pearlite reaction commences, the bainite reaction begins and both continue simultaneously. After holding the alloy steel at a temperature of 10504150 F. for the desired period, i.e., from 2 to 16 hours, a slight portion of the alloy steel may remain in its austenitic condition, although it has been severely depleted of carbon. As I commence to cool it at the very gradual rate as specified, any remaining untransformed low carbon austenite transforms into martensite as it approaches ambient temperatures. Thus my cooled alloy steel may show slight martensite in addition to pearlite and bainite. In no instances did I find any untransformed austenite in my coolalloy.

Further, my microstrncture exhibits a remarkable grain refinement over that which I described in connection with my aluminum addition earlier.

At this point in my process, I may machine the cooled casting to any desired dimension quickly and easily and without damage to any of the tools. The ease of handling, experienced with my alloy at this state, is believed attributed to the complete absence of untransformed austenite in my alloy. If no machining or shaping is desired, I proceed with my process in the manner I shall describe.

After transforming my steel alloy in accordance with at this temperature range for a period of about 1 but not more than about 4 hours, a period sufficient to bring the entire casting up to temperature. I then immediately and quickly cool the alloy, such as by quenching, thereby rapidly passing the alloy through its critical phase boundaries. This second heating coupled with my quick 1 quench completely reverses the transformation and. restores my alloy to a uniformly austenitic state. While in practice I prefer to utilize a water quench, I find that sections under about 2 inches can be air cooled and sections under 4 inches can be oil quenched without seriously disturbing the physical properties of my alloy.

In addition to the remarkable physical properties of my alloy, another salient characteristic is its highly refined grain structure. As can be readily appreciated from a consideration of Figs. 1 and 2 and in Table I below, the remarkable grain refinement of my alloy is vastly superior to any heretofore obtainable with austenitic manganese steels.

To further illustrate the remarkable properties of my alloy, I have prepared several heats which are identified in Table II, infra. For each heat, I prepared shouldered specimens having a diameter of 0.505 inch and a gauge length of 2 inches. Each of the samples was placed in a conventional Baldwin hydraulic tensile testing machine. With the Baldwin testing machine, I determined the tensile strength, the percent elongation in terms of the gauge length and the reduction in area as a result of the tensile forces placed upon the sample.

The compositions of these several heats, and the tensile strength, reduction in area, and elongation determined for each by my tests appear in Table II.

Table II Composition of Alloy Physical Properties Heat No. Tensile Elonga- Reduc- 0 Mn Or A] Ni Strength tion, tion of (p.s.i.) Percent Area,

Percent:

The remarkable combination of properties exhibited by my new alloy is readily apparent from Table II. The great significance of my discovery has been previously explained.

Brinell readings were also made for each heat and all averaged substantially 200, ranging from about 196 to 205. The close groupings of the Brinells for these alloys is believed to exist because my alloys are solid solutions and my composition range substantially precludes wide variation in solution hardening to any great degree. Without exception, all of my samples work hardened to a Brinell reading in excess of 500. i

.To further aid in the appreciation of the significance of the present invention, I have prepared and have set forth in Table III comparative work hardening data for standard 12 percent Hadfields manganese steel; for the special austenitic steel alloy from which Allis-Chalmers Manufacturing Company, West Allis, Wisconsin, makes their Mantalloy crusher parts (a steel alloy containing 1.1 to 1.3 percent carbon, 11 to 14 percent manganese, 2 to 2.5 percent chromium, and the balance essentially iron); and an alloy exemplifying the present invention (heat No. 2). In Table III, I have indicated for both the Mantalloy parts alloy and the alloy of this invention additional comparative figures indicating (1) the difference between the total length change of the standard manganese steel and the steel which is being compared with the standard grade and (2) the percent improvement calculated as one hundred times the difference in length change of the steel to be compared divided by the total length change of the standard grade.

Table III Number of 25 Foot-Pound Blows,

12% Manganese Steel:

Length (inches) 1. 000 0. 819 0.788 0. 760 0.740 0.715 0.705

Total Length Change 0.181 0.211 0.240 0.260 0.285 0. 29.5 Mantall' y, Parts Alloy:

Length (inches) 1.000 0.840 0.804 0.785 0.765 0.730 0.710

Total Length Change"... 0160 0.196 0.215 0.235 0.270 0.290

Difierencein Length Chan 0.015 0.025 0.025 0.005 0.005

Percent Improvement 11. 6 7.1 10.4 9. 6 1.7 1. 7 My Alloy:

Length (inches) 1.000 0.854 0.842 0. 835 0.815 0. 700 0.780

Total Length Chan 0.146 0.158 0.165 0.185 0.210 0.220

Difference in Length Change". 0.035 0.053 0.075 0. 075 0.075 0.075

Percent Improvement 19. 3 25.1 31. 2 28. 8 26. 7 25.4

I have further studied the equivalent deformation of standard 12 percent manganese steel and have compared with the Mantalloy parts alloy and the alloy of the present invention. These figures are reported in Table of austenite and a chemical composition consisting of 0.8 to 1.5 percent carbon, about 5.0 to- 9.0 percent manganese, 2.0 to 5.0 percent chromium, 0.05 to 0.15 percent aluminum, and the remainder essentially iron.

3. A fine grained austenitic steel alloy characterized All of the impact measurements reported in Tables III and IV were obtained by using a test bar sample as shown in Fig. 4 and subjecting it to repeated blows from a 50 pound weight being dropped from a heightof 6 inches. It is, of course, understood that any of a number of conventional test bars can be used to make these measurements.

As is readily apparent from this description, the methods and alloys herein described introduce remarkable advantages into the metallurgical art including means for obtaining machined austenitic steel articles and satisfy my aforestated objects to a degree beyond expectations.

It is of course understood that the procedures and alloys. described herein are intended to exemplify the present invention rather than limit it and that all modifications and variations falling within the spirit of this invention, especially as defined by the appended claims, are intended within its scope.

Having now particularly described and ascertained the nature of my said invention and the manner in which it is to be performed, I declare that what I claim is:

1. An alloy steel consisting of 0.8 to 1.5 percent carbon, 5.0 to 9.0 percent manganese, 2.0 to 5.0 percent chromium, 0.05 to 0.15 percent aluminum, and the remainder essentially iron.

2. A fine grained steel alloy characterized by a tensile strength of 100,000 to 140,000 p.s.i., an elongation of 16' to 44 percent (2 inch gauge), a reduction of area of 20 M46 percent; having a matrix composed substantially by a tensile strength of 100,000 to- 140,000 p.s.i., an elongation of 16 to 44 percent (2 inch gauge), a reduction of area of 20 to 46 percent and having a chemical composition consisting of 0.9 to 1.3 percent carbon, about 5.0 to 9.0 percent manganese, 2.0 to 5.0 percent chromium, 0.05 to 0.15 percent aluminum, and the remainder essentially iron.

4. A fine grained austenitic steel alloy characterized by a tensile strength of 100,000 to 140,000 p.si., an elongation of 16 to 44 percent (2 inch gauge), a reduction of area of 20 to 46 percent and having a chemical composition consisting of. 1.0 to 1.25 percent carbon, 6.0 to 8.0 percent manganese, 2.0 to 4.0 percent chromium, 0.05 to 0.15 percent aluminum and. the remainder essentially iron.

References Cited in the file of this patent UNITED STATES PATENTS 422,403 I-I'adfield Mar. 4, 1890 1,079,439 Potter Nov. 25, 1913 1,310,528 Hadfield July 22, 1919 1,975,746 Hall Oct. 2, 1934 2,206,847 Lorig July 2, 1940 OTHER REFERENCES Metals Handbook, 1948 edition, pages 526 534. Published by the American Society for Metals, Cleveland, Ohio. 

1. AN ALLOY STEEL CONSISTING OF 0.8 TO 1.5 PERCENT CARBON, 5.0 TO 9.0 PERCENT MANGANESE, 2.0 TO 5.0 PERCENT CHROMIUM, 0.05 TO 0.15 PERCENT ALUMINUM, AND THE REMAINDER ESSENTIALLY IRON. 